In a computer mail system, also known as an electronic mail system, a user may send messages to other users. Messages contain text or data in other forms. Messages sent within a computer mail system are typically transported by a computer network.
In order to enable the exchange of mail between a larger number of users, two computer mail systems may be connected by a "gateway." A gateway is a combination of hardware and software that routes messages between the two mail systems that it connects, thereby permitting users of either of the computer mail systems to send messages to users of the other computer mail system. Gateways further provide protocol, name and address translation services. Gateways may also enforce security restrictions that limit the addresses from which a routed message may be sent or at which a message may be received.
FIG. 1 is a network architectural overview diagram showing two mail systems connected by a gateway. The diagram shows mail system 110 connected to mail system 120 by a gateway 130. Mail system 110 includes four connected nodes, nodes 111, 112, 113, and 114. A node is a location in a mail system that may send or receive messages. Nodes of the mail system correspond to nodes of a computer network used to transport messages within the mail system. Messages may be sent within mail system 110 from any node to any other node. Mail system 120 similarly includes four connected nodes, nodes 121, 122, 123, and 124, between which messages may be sent. Besides being nodes of their respective mail systems, nodes 11 1 and 121 are also components of the gateway 130. Both gateway components are connected to common message stores, permitting them to exchange messages. The mail system 110 gateway component 111 receives messages sent from any of the mail system 110 nodes and addressed to any of the mail system 120 nodes. Such messages are known as mail system 110 to mail system 120 messages, or "ms110/ms120 messages" for short. The mail system 110 gateway component 111 transmits ms 11O/ms 120 messages via a communications link 119 to a transfer store 131 for msl10/ms120 messages. The ms 11O/ms 120 message transfer store 131 is therefore said to be the outgoing message store of the mail system 110 gateway component 111. The mail system 120 gateway component 121 retrieves ms 11O/ms 120 messages from the ms 11O/ms 120 transfer store 131 via a communication link 129, and transmits each to the mail system 120 node to which it is addressed. The msl10/ms120 message transfer store is therefore said to be the incoming message store of the mail system 120 gateway component 121. In the same way, the mail system 120 gateway component 121 stores messages sent from any of the mail system 120 nodes and addressed to any of the mail system 110 nodes ("20/ms110 messages") in a transfer store 132 for ms120/msl10 messages via communications link 129. The messages stored in transfer store 132 are then retrieved and transmitted to the mail system 110 nodes to which they are addressed via communications link 119 by the mail system 110 gateway component 111. Gateways may also provide additional services, such as translating messages from a format native to one mail system to a format native to another mail system.
FIG. 2 is a network architectural overview diagram showing a common alternative gateway implementation. Many network configurations permit stores located in one gateway component to be accessed by both gateway components, eliminating the need for a separate node between the mail systems to hold the stores. The diagram shows transfer stores 231 and 232 within the mail system 110 gateway component. In such an implementation, the mail system 110 gateway component is able to directly access the mail stores, while the mail system 120 gateway component accesses the mail stores via its connection 228 to the mail system 110 gateway component.
Even under the best of conditions, gateway components of the types shown in FIGS. 1 and 2, as well as others, are subject to failure. For example, a gateway component node may lose power, or experience an unrecoverable software crash. Further, the connection or connections between the gateway components may be disrupted, either by a physical failure of the connection or by an error condition of the connection. In these failure situations, the gateway ceases to perform its function, preventing any further exchange of messages between the mail systems connected by the gateway. In a typical gateway installation, few people have the expertise needed to remediate gateway failure. For example, at some installations, only a gateway administrator, responsible for maintaining the gateway, or a mail system administrator, responsible for maintaining one or more of the connected mail systems, have the necessary expertise.
However, gateway failure is usually first detected by a user of one of the mail systems who sends a message to a node of the connected mail system, then discovers by communicating with the intended recipient of the message using a mode of communication other than computer mail that much more than the average delivery time has elapsed without the delivery of the message. The user then must contact a gateway or mail system administrator in order to notify the administrator of the failure in order for the failure to be remediated. As computer mail becomes an increasingly prevalent communication medium, several important disadvantages of user detection of gateway failure become apparent. First, it is time consuming for a user to take responsibility to monitor the progress of the messages that he or she sends. Second, users that frequently detect gateway failure may lose confidence in computer mail as a reliable communication medium. Third, failure remediation is limited by the length of time it takes for a user to notice the nondelivery of an inter-mail system message, which can be considerable. As the time interval between failure and failure remediation grows, so does the number of inter-mail system messages whose delivery is delayed. Additionally, as additional delayed inter-mail system messages accumulate, the length of time required for the gateway to exchange the accumulated delayed messages following remediation increases. Fourth, failure remediation is also limited by the availability of users to send and monitor inter-mail system messages. For instance, users may not be available to perform this function late at night or on weekends and holidays.